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1275 Cranks
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Author:  kirby [ Thu Apr 19, 2012 3:32 pm ]
Post subject:  1275 Cranks
I have bought a bare 1275 block of the 1100s auto variety and I'm trying to work out what type of crank it needs.

Can someone clear up the differences,
I keep reading about thick and thin tail cranks, Which I assume refers to the mains size? are the thick tail cranks S block only?
Apparently early 850 cranks can be used why, how any benefit?
Anything else I should know?

Oh and if anyone has a set of suitable crank and rods for sale let me know.
Author:  simon k [ Thu Apr 19, 2012 3:40 pm ]
Post subject: 
nah, forget about thick and thin tail cranks - you need an 1100S or A+ 1275 crank and that's all there is to it. Edit: Once you have located a crankshaft, it's quite easy to find conrods to suit the bigend size

The difference with 1275 cranks (where you may have become confused) is small and large journal, which is a difference in the bigend size. The other difference between 1275 cranks is that Cooper S cranks have a thinner centre main, so won't actually go into an 1100S block, though an 1100S crank will go into an S block with the use of thicker thrusts (or so Dr Mini told me once, which I may have been confused about!)

A thin tail crank is either a really early 850, that actually has a smaller taper for the flywheel, or a small bore (later 850/998/1098) crank that has a smaller diameter where the gear part of the primary gear runs - but in any case, all varieties of those cranks have smaller main journals, so won't go in your block anyway
Author:  drmini in aust [ Thu Apr 19, 2012 7:27 pm ]
Post subject: 
Simon,
A Cooper S crank WILL go into an 1100S block, my 1360 runs one.
You just surface grind ~.016" off the steel backing of the standard thrusts.. :wink:

Yes as I told you before, if you put an 1100S crank in an S block you use +.030" thrusts and surface grind their steel backing to suit.

You want at least .006" end float if it's getting a rev, .003" in the book is not enough. I found out the hard way...

Kirby, re the rods, small journal 1100S and Cooper S (most 1275) ones are the same. AEG521.

If running a big journal 1275 crank (unstroked) you want late 1100S rods, or A+ rods (lighter & better).

Stroking the big journal 1275 cranks by ~3mm is popular, it's an easy way to pick up ~50 extra cc and more torque. You then need Cooper S rods (narrowed in width by 1/32" approx).
I'm building one at the moment...

Of couse if you have the do$h, you could buy aftermarket competition rods but IMO they are bit overkill for street use.
Author:  simon k [ Thu Apr 19, 2012 9:26 pm ]
Post subject: 
drmini in aust wrote:
Simon,
A Cooper S crank WILL go into an 1100S block, my 1360 runs one.
You just surface grind ~.016" off the steel backing of the standard thrusts.. :wink:


gotcha - makes sense :)
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